1. Field of the Invention
The present invention relates to a process for producing vinyl chloride-based polymers, and particularly to a process for producing vinyl chloride-based polymers by which vinyl chloride-based polymers of high quality can be produced with high productivity.
2. Description of the Prior Art
In general, suspension polymerization of vinyl chloride or a vinyl chloride-containing monomeric mixture (hereinafter referred to as "vinyl chloride or the like") is carried out, for example, by a process as follows. First, a polymerization vessel equipped with a jacket and a reflux condenser is charged with an aqueous medium, a dispersing agent (suspending agent) and a polymerization initiator. Then, after evacuation of the polymerization vessel, the monomer is charged into the vessel, optionally with other various additives as required. Thereafter, hot water is let flow through the jacket for heating until a predetermined reaction temperature is reached in the vessel. Subsequently, cooling water is let flow through the jacket to remove the heat of polymerization reaction, thereby maintaining the predetermined reaction temperature in the vessel while the polymerization reaction is conducted.
In recent years, for attaining higher productivity, attempts have been made to shorten the time required for each batch of polymerization. For shortening the polymerization time per batch, decreases should be made in the time required for raising temperature, as well as in the time necessary for charging materials into the polymerization vessel, the reaction time, etc.
As a method of shortening the temperature-raising time, there have been known, for example, (1) a method wherein an aqueous medium with a suspending agent dissolved therein which has been preliminarily heated to a predetermined temperature and a monomer with a polymerization initiator dissolved therein which has been preliminarily heated to a predetermined temperature are simultaneously charged into a polymerization vessel (Japanese Patent Publication (KOKOKU) No. 60-26488 (1985)); (2) a method wherein a monomer and a polymerization initiator are charged into a polymerization vessel and mixed uniformly with each other, and then water which has been heated to a temperature of 50.degree. to 80.degree. C. and deaerated is charged into the vessel (Japanese Patent Publication (KOKOKU) No. 1-50243 (1989)); and (3) a method wherein at least 70% by weight of a monomer to be charged and at least 70% by weight of preliminarily heated water to be charged are simultaneously charged into a polymerization vessel and wherein a suspending agent and a polymerization initiator are charged into the vessel at controlled timings (Japanese Pre-examination Patent Publication (KOKAI) No. 1-172407 (1989)). These methods are not only effective for the purpose intended but are excellent in view of compensating for the deficiency of heating capability of the polymerization vessel, which deficiency arises from increases in vessel size and the use of reflux condensers.
However, although the above-mentioned methods are extremely efficient in shortening the temperature-raising time, they involve complicated operations for charging the necessary materials into the polymerization vessel. In addition, the methods necessitate mixers for preparing the liquid materials to be charged (for example, the aqueous medium with a suspending agent dissolved therein which is preliminarily heated to a predetermined temperature, as described in (1) above) and equipment for heating.
Furthermore, the above method have the disadvantage that the polymeric product obtained includes many coarse grains, and has an increased number of fish eyes when formed into a sheet or the like. This is probably because the inside of the polymerization vessel is heated to a high temperature before the monomer charged into the vessel becomes suspended as oil droplets uniformly and stably in the aqueous medium, and, as a result, polymerization reaction is initiated and proceeds too rapidly.